1. Field of the Invention
The present invention relates to the preparation method for polyurethane foam. In more detail, the present invention relates to the preparation method for low-density rigid polyurethane foam excelling in the flame retardance and the dimensional stability.
2. Description of the Related Art
Prior art is Japanese Patent Application Laid-open No. 2002-020444.
Rigid polyurethane foam excels in insulation characteristics and moldability among other properties and is thus widely used as insulating materials or structural materials for the housing, the cold storage and the like. Said foam is generally prepared by reacting suitable polyisocyanate with polyol in the presence of the blowing agent. Hydorchlorofluorocarbon (HCFC) is mainly used as the blowing agent for the preparation of rigid polyurethane foam, but HCFC may not be used in the future due to the depletion of the ozone layer. Furthermore, hydrofluorocarbon (HFC) is recently proposed as an alternate candidate for HCFC which may not deplete the ozone layer, but there exists the shortcoming of greater green house effect. Hydrocarbon can be mentioned as the blowing agent other than HCFC and HFC, but they can not be said to be practical as they possess the explosive flammability and pose the safety problem awaiting solution. Therefore, the general tendency has been to use carbon dioxide generated in the reaction between water and polyisocyanate as the blowing agent. However, when rigid polyurethane foam is prepared by using such carbon dioxide alone, there exists the shortcoming of foams being easily shrinked as the rate of diffusion toward the outside of the foam for carbon dioxide within the formed foam is greater than the rate of influx for air into the foam, thus reducing the internal pressure of the foam therein. Means for solving such problem are known such as increasing the strength by increasing the density of rigid polyurethane foam or by opening a part of cells. However, increasing the density is not only ineconomical but also leading to the increase in fuming quantity, so it becomes difficult to pass the Grade 3 incombustibility test specified according to JISA1321. On the other hand, it is economical to make the interconnecting cell rather than the closed-cell as the insulating materials because the thermal conductivity immediately after the preparation is 0.023 mW/(m·K) becoming to 0.035 mW/(m·K) due to displacement in a short time of carbon dioxide in the foam by air for the closed-cell of rigid polyurethane foam prepared with carbon dioxide generated in the reaction between water and polyisocyanate as the sole blowing agent. However, the water vapor permeance is large at the closed-cell content of less than 50%. Moreover, the volume of water required becomes greater because more carbon dioxide is required in order to decrease the density. Thus, rigid polyurethane foam to be formed becomes friable and the adhesion tends to become weaker with the formation of excess urea bond. As such, there exist various problems as to forming rigid polyurethane foam by using solely carbon dioxide generated in the reaction between water and polyisocyanate.
Therefore, in recent days, the technology wherein combining carbon dioxide under supercritical fluid, subcritical fluid or liquid state in addition to carbon dioxide generated in the reaction between water and polyisocyanate as blowing agent is proposed in order to further improve properties of rigid polyurethane foam to be formed.
For example, Japanese Patent Application Laid-open No. 2002-020444 discloses the production method with an ultrafine-cell foam production apparatus equipped with a plurality of pumping means for metering and delivering raw material components necessary for producing a polyurethane or polyisocyanurate foam, a plurality of transport means for transporting the raw material components delivered by the pumping means to the site of object, and a mixing chamber for mixing the raw material components, wherein at least one raw material component flowing through the transport means is set at 20×102 kPa or higher and that a gas is continuously dissolved into the raw material component flowing through the transport means under a pressure higher than that of the component. The preferred embodiment among them is such that said gas is CO2 gas that is also injected into the raw material component flowing through the transport means under the pressure more than the critical pressure. Furthermore, the apparatus wherein each of a tank holding polyisocyanate component and a compounding tank holding polyol component, foam stabilizer, catalyst and the blowing agent (water) is connected to the mixing chamber through piping is described in Examples, but it is so contrived that water as the blowing agent is compounded into the polyol component beforehand and stored, and its mixture is transported to the mixing chamber through piping at the reaction time with the polyisocyanate component.
The formation of an ultrafine-cell foam to a extent of a few μm/piece to several tens μm/piece by using carbon dioxide under subcritical fluid, supercritical fluid or liquid state as the blowing agent is only described in said publication.
On the other hand, the foam giving good dimensional stability even at high temperature and low temperature is preferred as rigid polyurethane forms are widely used as the insulating materials or structural materials for the housing, the refrigerator and the like. However, as the configuration of cells in general is in the form of the anisotropy extending toward the rise direction, and the strength between the rise direction and its perpendicular to rise direction differ, there is a tendency wherein the larger the anisotropy of cells, the lower the dimensional stability.